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SHUTTING Out the NOISE
ilots on the flight deck need to renders the other impossible to hear. Noise-induced hearing loss can be either hear what they want to hear as Low frequency noise — the sound pro- temporary or permanent. A temporary much as they need to be pro- duced by the slipstream, for example hearing loss is a brief shift in the audi- tected from what they don’t want — masks higher frequency sounds tory threshold that occurs after a rela- Pto hear. While visual information is such as an instruction from air traffic tively short exposure to excessive noise predominant on flight decks, com- control, a crewmember’s instructions or — more than 90 decibels (dB). Normal plete reception and comprehension of an audio warning. hearing recovers fairly quickly after the spoken words from air traffic control- Task performance may be affected noise stops. However, if the noise level is lers and other crewmembers and audio by the presence of noise, especially sufficient to damage the tiny hairs in the signals from electronic systems are vital loud noise. Studies suggest that noise cochlea — the part of the inner ear that to flight safety. Aviation headsets — es- reduces a person’s overall accuracy is responsible for transforming sound pecially with recent advances in noise and negatively affects the completion waves into the electrical signals that go reduction and sound attenuation — can of complex tasks1 — tasks that require to the brain — the threshold shift can enhance a pilot’s ability to hear those attention to a large number of cues pre- be irreversible, resulting in permanent words and signals throughout flight sented concurrently or in succession.2 partial or total hearing loss. while also protecting hearing. Noise can produce a host of reac- tions in humans. These can be physi- Three Components Consequences of Noise ological, such as headache, fatigue, On the flight deck, headsets — a Flight deck noise affects both the per- nausea and insomnia; psychological, considerable improvement over hand formance and safety of flight crewmem- such as irritability, anger and anxiety; microphones and speakers once widely bers, and can induce hearing loss. or cognitive, such as impaired concen- used by pilots — protect hearing and Noise on the flight deck affects tration and decreased ability to estimate reduce ambient noise while enhancing communication by masking important elapsed time. critical communications. audio signals. Masking is a process in Among the worst consequences Nathaniel Baldwin is credited with which one of two simultaneous sounds of high levels of noise is hearing loss. developing the first basic radio headset
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Technological advances have resulted in headsets that enhance communication and protect pilots’ hearing.
BY CLARENCE E. RASH SHUTTING Out the NOISE
while he was a student at Stanford Uni- Conventional passive noise reduc- enough energy to make the cup vibrate versity in California, U.S., and patenting tion consists of putting something and therefore are not heard. it in 1910. There was little interest in the — such as earplugs or earmuffs — in or ANR, first conceived in the 1930s device until the eve of World War I, when over the ears to block noise. Properly and refined in the 1950s, did not the U.S. Navy ordered 100 headsets.3 inserted, standard earplugs typically become prevalent in aviation until the Nearly all headsets have three basic provide about 30 to 35 dB of protection 1990s.4 In conventional ANR head- components: a microphone, ear cups across the entire noise spectrum. Most sets, the frequency and amplitude of or earplugs, and a harness or head- modern passive over-the-ears headsets the sound inside the headset cavity band. Most current aviation headsets use soft and flexible cushions to form are measured by a small microphone, are classified as either “heavyweights” a good seal, preventing ambient noise and a 180-degree out-of-phase copy is — which clamp over the user’s head, from directly entering the ear and pro- produced and fed back into the headset; covering both ears — or “lightweights,” viding 20 to 30 dB of protection, which the two signals superimpose and cancel which resemble personal music ear- is most effective at higher frequencies. each other. phones. Lightweights, while designed Anything that interferes with the seal, Regardless of the type of feedback to be more comfortable, typically do such as sunglass or eyeglass temples, loop used, in order for ANR headsets not perform as well as heavyweights in compromises the noise-protection to perform their task, they require blocking out aircraft noise. level. Sounds are conducted toward the electrical power from either a battery Another method of classification is ear when sound waves — such as the pack or the aircraft. The ANR technol- based on the hearing-protection tech- relatively high-frequency sound waves ogy is generally effective only at lower nology incorporated into the headset. produced by voices or audio signals frequencies. A protection level of 20 The two types of technology currently — hit the outer ear cup and cause the dB over this lower frequency range is used are passive noise reduction — or cup to vibrate; this, in turn, causes the typical. This makes this type of headset attenuation — and active noise reduc- air inside the cup to vibrate. Low- most effective in environments such tion (ANR) — also called electronic frequency sound waves such as those as aviation, where the noise spectrum noise cancellation. produced by the slipstream do not have consists of mostly lower frequency © Copyright 2006 iStock,© Copyright International Inc.
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sounds.5 ANR headsets also provide headset communications. An approach about twice the cost, ANR headsets are signal-to-noise improvement — clear that provides the extra protection of considered superior to passive noise re- sounds — by reducing the masking ef- the earplug without compromising duction systems. Nevertheless, perfor- fects of low-frequency noise. communication is the communication mance depends greatly on the ability of However, ANR is not a one-size- earplug (CEP). the headset ear cups to seal over the ear. fits-all system. Every pilot has a differ- The CEP, which initially was devel- A label should disclose the headset’s ent hearing response. Therefore, the oped for use in U.S. Army helicopters, protection level — a noise reduction perceived performance of a selected provides the high-quality hearing pro- rating of at least 24 dB is recommended ANR headset varies from pilot to pilot. tection of an expanding foam earplug (see “Rating Systems”). Ultimately, the best method of evaluating while allowing clear passage of speech Although the FAA does not regulate the effectiveness of an ANR device is for communication to the ear. A miniature headsets, it does provide specification the pilot who plans to use it to try it out in speaker and foam earplug are coupled guidance in two technical standard the aircraft that he or she usually flies. to yield a lightweight communications orders (TSOs).8,9 Many of the specifica- ANR aviation headsets were de- device that can be used alone or in tions discuss construction and environ- veloped to cancel the lower frequency combination with over-the-ear hear- mental criteria for the manufacturer sounds produced by piston engines and ing protection. The CEP, when worn rather than operating and protection propellers. Because turbine engines in combination with other hearing performance criteria for the user. typically produce sounds in the mid- to protection, reduces noise exposure to Nevertheless, headsets that meet these high-frequency ranges, ANR headsets minimal levels. specifications bear the applicable TSO designed for use in piston aircraft will Tests conducted by the Army marking — and this should be a factor be less effective in turbine aircraft. showed reductions of “more than 30 in the selection process. Ideally, a detailed knowledge of the dB for the low frequency noise spectra Another performance factor is how noise spectrum of the specific aircraft, that are prevalent in helicopters,” said a headset receives its power. A headset compared with the performance of the Ben Mozo of Communications and Ear that can be operated with either battery ANR headset, will allow each pilot to Protection Inc., developer of the device. power or aircraft power is a plus. choose the ANR device that is best for In addition to active/passive him. Unfortunately, the noise spectrum Selection Criteria technology, overall comfort should be pattern produced by a specific aircraft A quick survey of the market reveals considered. Weight and “feel” — that is, is rarely available.6 more than 25 manufacturers offer- clamping pressure and headband/strap The U.S. Federal Aviation Adminis- ing more than 100 different aviation design — determine how comfortable a tration (FAA), as well as many hearing- headset models that range in price from headset will be, especially after several protection organizations, recommend about US$100 to more than $1,000. hours of flight. Headset weights typi- a dual protection approach, using both In addition to price, headset selec- cally are about 12 to 18 ounces (340 to earplugs and a headset.7 However, tion criteria can be separated into per- 510 grams); a difference of a few ounces the use of earplugs under a headset formance, comfort and added features. may not seem worth worrying about, is somewhat counter-productive; the For performance, the first decision but over a long period, the extra weight earplugs impede a pilot’s ability to hear is active or passive noise reduction. At can induce neck strain and headache.
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From left, Lightspeed Aviation’s LightFlight Mach 1 Headset, Lightspeed’s Twenty 3G, Communications and Ear Protection’s communications earplug, Lightspeed’s QFRXCc and David Clark Co.’s X11.
Clamping pressure and the monaural speakers, or individualized Clarence E. Rash is a research physicist at the headband/strap mechanism also are speaker controls on each headset ear U.S. Army Aeromedical Research Laboratory in Fort Rucker, Alabama, U.S. He has more important considerations. All headsets cup to allow for independent adjust- than 25 years of experience in Army aviation of reasonably good quality have some ment of the volume for each ear. research and development and is the editor of type of adjustment to permit expan- However, the stereo option is useless “Helmet-Mounted Display: Design Issues for sion or tightening of the ear cups. The if the aircraft’s audio system does not Rotary Wing Aircraft,” SPIE Press, 2000. amount of padding on the ear cups support stereo outputs. and headband/strap is a strong indica- As with most electronic equip- Notes tor of how comfortable the headset ment, keeping a headset in good work- 1. Miller, J.D. “Effects of Noise on People.” will be. ing order requires regular preventive Journal of the Acoustical Society of America Finally, if possible, the pilot should maintenance. Exterior parts should be Volume 56 (1974): 729–762. test the headset in the aircraft that he or cleaned regularly with mild soap and 2. Kahneman, D. Attention and Effort. she usually flies. water, and dried well before use. Peri- Englewood Cliffs, N.J., U.S.: Prentice Hall, Most headsets offer additional odic inspections should be conducted 1973. features. One option is a choice of ear to check for cracks on ear cup seals 3. SKC Communication Products. A Brief seals made of foam plastic or newer and missing parts, and to ensure that History of Headsets.
5. Gauger, D. “Should You Consider ANR for Hearing Protection?” The Newsletter Rating Systems for the Council for Accreditation in Occupational Hearing Conservation, Volume 15 (2000). ne method of rating the protection level provided by hearing protec- tion devices is the noise reduction rating (NRR) system specified by the 6. Kernstock, N.C. “Headsets, Helmets and OU.S. Environmental Protection Agency. It is the measure, in decibels (dB), Communications: A Buyer’s Market.” of how well a hearing protector reduces noise; the higher the number, the Rotor and Wing. May 2001. greater the noise reduction. When dual protectors are used, the combined NRR is usually accepted as 7. U.S. Federal Aviation Administration 5 dB more than the higher rating of the two devices. For example, if a pair of (FAA). Pilot Safety Brochure AM-400- earplugs with an NRR of 25 dB is worn under earmuffs with an NRR of 23 dB, 98/3, Hearing and Noise in Aviation. 1998. the NRR of the combination is 30 dB (25 dB + 5 dB = 30 dB). 8. FAA. Technical Standard Order (TSO) Additional protection rating numbers are in use throughout the world. TSO-C57a, Headsets and Speakers. 1983. These include the single number rating (SNR) system defined by International Organization for Standardization in ISO/DIS 4869-2.2. 9. FAA. TSO-C139, Aircraft Audio Systems — CER and Equipment. 2005.
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